A method and a diagnostic device for leak testing of a tank system of a motor vehicle is described in U.S. Pat. No. 5,263,462. During a shutoff phase of the motor vehicle, the tank system is sealed off using a solenoid valve and, during a longer diagnostics time, the pressure and temperature characteristics in the tank are detected using a sensor system and transmitted to the control unit.
If a partial vacuum builds up during the diagnostics time while the tank cools off, the air tightness of the tank system is inferred therefrom.
This procedure provides that the control unit must be switched on during the entire diagnostics time. In addition, the solenoid valve must also have power supplied to it continuously during the diagnostics time. This requires a high energy consumption during the shutoff phase, which significantly loads the electrical system of the vehicle. These problems may be counteracted by a higher performance configuration of the vehicle electrical system, which, however, causes higher costs. The diagnostics period may also be reduced, which in turn reduces the reliability of the diagnosis.
In a mechanism, previously disclosed by Siemens and DaimlerChrysler in SAE Toptec 99, Indianapolis, the tank is sealed off from the environment by passive non-return valves, which close without current and, therefore, do not consume any energy in the sealed state. However, if the tank is heated, overpressure builds up in the tank during the shutoff phase which, if there is a leak, results in fuel gas and/or vapor, which contains hydrocarbons, being able to escape from the tank system into the environment. In addition, a detection electronic system must also have current supplied to it continuously during the entire diagnostics time in this case.
Furthermore, German Published Patent Application No. 100 13 347, which is expressly incorporated herein in its entirety by reference thereto (e.g., to FIG. 3 therein and the description), a device for low-emission operation of a fuel tank system is described which includes, among other things, an activated carbon filter for dissipating excess fuel gases, a controller, and, e.g., a currentless bistable solenoid valve, which remains in the closed state and in the open state without current. A current pulse is only necessary for switching between these two states. The controller is electrically connected to a first pressure sensor positioned inside a fuel tank and a second pressure sensor positioned outside the fuel tank. Corresponding temperature sensors are also indicated as alternatives. From the pressure data acquired, the controller evaluates whether a partial vacuum or overpressure is to be expected in the tank after the vehicle is turned off. In the case when the analysis of the pressure data indicates that, after the vehicle is turned off, an overpressure in relation to the ambient pressure is to be expected, the solenoid valve is opened in order to conduct the excess fuel gas into the environment via the activated carbon filter and therefore with low emission. In contrast, in case of an expected partial vacuum, the solenoid valve remains closed, after which the leakage test using the partial vacuum may be performed in accordance with the procedure described in U.S. Pat. No. 5,263,462.
Using this device, the emission problem described above is largely eliminated. In contrast to U.S. Pat. No. 5,263,462, the power consumption of the solenoid valve is additionally avoided. However, the controller must be active for the entire diagnostics time.